Hi, I feel that I'm a quite experienced Linux user, but I overlooked that your linux commandline to create a usb stick contained the target device twice - the first one was the relevant one.
I actually overwrote my /dev/sdb now. Luckily a backup drive only. I strongly recommend to split your install command into one line per actual command and highlight BOTH (!) occurrences of the target device, so that little tired people like me dont mess up their system. Thanks.
sudo dd if=Solus-4.0-Budgie.iso bs=1M of=/dev/sdb
sudo sync
sudo eject /dev/sdb